Aircraft in flight are constrained to operate within certain parameters. For example, there is an airspeed which must never be exceeded, usually called V.sub.ne. There is also an airspeed which must not be exceeded except in smooth air, usually called maneuvering speed. Further, there is an airspeed (which is a function of dynamic wing loading) below which the wing stalls and the airplane falls. These airspeeds are presently indicated by colored markings on the airspeed indicator and on placards within the airplane. The pilot is responsible for observing these limitations throughout the operation of the airplane. In particular, the pilot must estimate, without direct indication, the possibility that the airspeed may be near stall or structural damage.
Similarly, the airplane is subject to vertical acceleration forces resulting from climbing, descent, and turbulence. The pilot is required to keep these forces below levels which will cause structural damage. This result is accomplished by restraint in manipulation of the controls above maneuvering speed and by allowing the wing to stall below maneuvering speed. It is important to note that stalling is an undesirable event, but it is preferable to structural damage. A small number of aircraft are equipped with accelerometers to indicate the level of these forces, but normally pilots receive no quantified indication of acceleration.
At present there is no operational tool for indicating the operating point of the airplane in flight with respect to the operational limits, or the "envelope", of airspeed and vertical acceleration. In many cases, the instruments operate independently, leaving it to the pilot's judgment to relate these parameters. In other cases, the parameters may be measured and related by the instruments, but the display of information to the pilot is insufficiently related to the maneuver envelope. See, for example, U.S. Pat. Nos. 5,359,326 and 4,590,475.
It is, therefore, very desirable to develop devices and methods which indicate to a pilot of an aircraft where the operating point of the aircraft is in relation to the aircraft's operating limits.